Friday, October 18, 2013

McIntosh MC275 Tube Amplifier

From their website:

"An object of desire cherished by music lovers around the world, the MC275 was
designed and engineered in 1961 by McIntosh co-founder Sidney Corderman and the
McIntosh Engineering Team. Based upon the extraordinary Unity Coupled Circuit,
the MC275 has lush sound that
presents a powerful and detailed soundstage. Combining a polished Stainless
Steel chassis with the latest vacuum tube circuit layout advancements move MC275
solidly into the 21st century. This amp gives new dimension to sound
quality."

"The 50th Anniversary Limited Edition McIntosh MC275 combines modern convenience
and performance enhancements without altering the sound character that has made
it famous for the past 50 years. A very limited quantity of the special 50th
Anniversary Limited Edition McIntosh MC275 has been created for only the most
discerning of audio connoisseurs. Individuals who are fortunate enough to own
the 50th Anniversary Limited Edition are certainly amongst the most
sophisticated audio enthusiasts globally."

The MC275 has all the features to
meet the needs of today’s music enthusiast, while preserving the qualities that
have made it the most revered McIntosh tube amplifier.

This sixth generation MC275 has many subtle enhancements from the previous
release that greatly improve performance. Combined with the widest bandwidth
McIntosh output transformer design, the MC275 features tauter bass control and
thermally quiet circuit design. The result is pure music.

The seven small tubes are LED illuminated to indicate the real-time status of the warm-up process: upon power
up, they sequentially glow amber for a few seconds and then all turn green to
signify system readiness. In the rare case of an output tube
failure, the small tubes in front of it will turn red to notify you of the
problem. Power Control is used to remotely control tube illumination from a
McIntosh preamplifier.

I hate to say this, but I believe, now, after finally hearing this awesome instrument last night, that I've been to the mountaintop. I never thought a singular event such as this would so color (pardon the audio pun) my perceptions of what the best (cost versus performance) sound could actually be, that I actually figured I might never achieve the experience, much less ever own this piece of audio history. I WILL own one someday, and I DO believe the hyperbole expressed in the press blurb I quoted earlier. The game has changed until the 275 is a reality for me. No more lateral moves, amp-wise.

Here's the tech stuff, from Audio Classics:

This Amplifier was produced by McIntosh from May, 1961 through July 1973. The original Unity Coupled Circuit and McIntosh bifilar wound output transformer set a standard for low distortion performance back in 1947. The output transformer had two primary windings which are wound in a bifilar pattern. In this technique, both primaries are wound side by side. Each turn of primary number one is next to the same turn of primary number two. There is near complete magnetic coupling between the two wires. The magnetic coupling is reinforced by the capacitance between the two wires.

In the McIntosh Unity Coupled Circuit, one of the bifilar primary windings is connected through the power supply to the plate and cathode of one of the output tubes. The other bifilar winding is connected to the other tube in the same manner. This unity coupled output virtually eliminates notch distortion.

The MC275 used an advanced design of this technology by incorporating a trifilar wound primary to improve the power bandwidth. The third winding is connected to the plates of the cathode follower driver. Groups of interleaved primary and secondary windings improve coupling and bandwidth of over 100,000 cycles is achieved.

Harmonic Distortion is less than 0.5% at rated output or less for 20Hz through 20kHz. Typical performance is 0.3% or less at 20Hz and 20kHz full power. Typical performance in the mid frequencies is 0.1% or less at full power.

Output barrier strips provide taps for speaker impedances of 4, 8 and 16 ohms stereo or 2, 4 and 8 ohms mono. The terminal strips may also be connected for a constant voltage output of 25 volts in either stereo or mono, or for 70.7 volts in mono. The secondary of the McIntosh output transformer may be operated either grounded or not grounded. If grounding is desirable, a wire may be connected between chassis and common of either or both output terminal strips.

An octal socket marked line output has connections for 70.7volts, 115 volts, 230 volts or 600 ohms for stereo 300 ohms for mono operation. The 600 ohm winding and the 300 ohm winding are balanced to ground. Pin 1 of the octal socket is the center tap for these windings and is grounded.

And more:

This Amplifier was produced by McIntosh from 1993 until 1996 to commemorate the late Gordon J. Gow, McIntosh President from 1989 until 1997, and Vice President since the inception of the company in 1949. Mr. Gow was the co-inventor of the original McIntosh Unity Coupled Amplifier circuit. He was also the person most responsible for the unparalleled success and reputation of McIntosh Laboratory as the manufacturer of the world's finest audio electronics.

The overall appearance of the commemorative MC275 amplifier is very close to the original. It has the same dimensions, chrome chassis and black transformer housings. Several changes were made to the original design to take advantage of newer technology and accommodate the needs of contemporary sound systems.

Gold plated XLR balanced input connectors were added on the sloping section of the chrome chassis. This allows the use of balanced interconnect cables for lower noise and improved signal transfer. The amplifier sensitivity at the balanced input jacks is fixed at 2.0V for rated output.

An increase in gain was necessary to accommodate the balanced inputs. Circuit changes were made for this purpose, resulting in the change of four vacuum tubes. Each 12BH7 was replaced by a 12AZ7 and each 12AU7 was replaced by a 12AX7.

Two gold plated RCA unbalanced input jacks were placed on the front of the chrome chassis. Above these input jacks on the sloping section of the chassis are corresponding left and right volume controls. The sensitivity at the unbalanced inputs, with controls set at maximum, is 1.0V for rated output.

Two slide switches were placed below the balanced input connectors. One switch selects balanced or unbalanced inputs. The other switch selects stereo or mono parallel configuration.

Gold plated high current output barrier strips were placed on the sloping section of the chrome chassis. Output taps were provided for speaker impedances of 4, 8 and 16 ohms stereo or 2, 4 and 8 ohms mono.

The octal output socket that originally connected to the high impedance (500 ohm) transformer tap was removed. The 3 position lever mode switch, stereo balance control, twin amp volume controls, mono volume control and the twin amp input jacks were also removed.

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